A number of studies have demonstrated the importance of the cooperation between macrophages and lymphocytes in the development of a competent cellular immune response. Macrophages appear to play an important role not only in initiating the immune response by "processing" antigen for presentation to lymphocytes, but also by performing a critical function as effector cells by their interaction with soluble mediators produced by stimulated lymphocytes. Previous studies of the role of macrophages in the immune response have been performed using peripheral blood macrophages (monocytes) or macrophages present in inflammatory exudates. There appear to be fundamental differences in these macrophage populations and long lived tissue macrophages. Since the initial contact that an inhaled antigen has in the lung would apparently be with resident alveolar macrophages (PAM), it is important to investigate whether these cells have the ability to participate in cellular immune reactions. Guinea pig PAM will be harvested by standard techniques and established in monolayer cultures. The ability of PAM to respond to lymphocyte mediators will be evaluated by studying morphology, RNA and DNA synthesis, cell adherence, and rates of phagocytosis when these cells are cultured for varying periods of time in lymphocyte mediator rich media. The ability of these cells to process antigen will be evaluated by studying their ability to augment proliferation of purified lymphocytes when cultured with a sensitizing antigen.